It is now common to process logs into wood chips at logging sites rather than transport the unprocessed logs to a processing mill for chipping. It is easier and more cost effective to haul chips from the logging site rather than haul unprocessed logs.
The processing of logs into chips at the logging site requires apparatus to first debark/delimb the logs and then reduce the debarked/delimbed logs into chips. The standards for the chips produced is changing. The allowed percentage of contaminants in the chips such as bark and other debris is decreasing. This means that the debarking/delimbing operation must provide a "cleaner" log before it is chipped. Not only must the log be clean, but loose bark and other debris must be prevented from entering the chipping machine. Along with the higher standards for chips, many mills of today require a chip that is larger in size.
A combination machine was developed that had a debarking/delimbing machine, chipping machine and loader mounted on a single chassis. A machine of this type is disclosed in U.S. Pat. No. 33,840, Peterson et al (Reissue of U.S. Pat. No. 4,889,169). The combination machine has the capability to process multiple logs at once and most often bundles of logs are fed into the machine. The chipping machine is mounted at a distance from the debarking/delimbing machine and a conveyor is provided to convey the log bundle exiting the debarking/delimbing machine into the chipping machine and to provide an exit for the loosened bark. The machine has a longitudinal feed path that extends substantially along the length of its chassis. The debarking/delimbing machine has a lower flail in a fixed position and an upper floating flail that will adjust to the largest diameter log of a log bundle.
One of the problems with the processing of logs in bundles is that the logs diameters vary from log to log. The upper flail of the debarking/delimbing machine adjusts to the largest diameter log in the bundle. On occasion a log in the bundle will not be adequately contacted by the upper flail and more than an acceptable amount of the bark will not be loosened or removed. The lower flail has gravity working on its side and regardless of the log diameter the bottom side of the log will be impacted by the lower flail to loosen and remove the bark, limbs and other debris from the log. Once the log passes the flails there is no mechanism to loosen the bark. The bark retained on a log will be chipped with the log as it enters the chipping machine and thus will contaminate the chips. Also, even with the chipping machine spaced at a distance from the debarking/delimbing machine, loosened bark may be transported on top of the bundle into the chipping machine. Contamination of the wood chips whether it be from intact or loosened bark is to be avoided to the extent possible.
As was previously mentioned, mills often require chips that are larger in dimension than chips previously used. One of the ways of increasing the size of the wood chips is to increase the rate at which the logs are fed into the chipping machine. This requires that the logs be fed through the debarking/delimbing machine at a higher feed rate. This compounds the bark removal process. In order to accommodate the higher feed rates, the rotational speed of the flails must be increased. This has a detrimental effect in that the life of the flails (chains) is dramatically decreased. One of the highest maintenance costs of the debarking/delimbing machine is the cost of replacing the chains due to wear or breakage. It is also believed that the increased speed required of the flails will often damage the fibers of the log exposed to the impacting of the flail. The damaged fibers result in a chip of poorer quality.